import java.util.*;
public class gjtgst {
    public static void main(String[] args) {
        int  a[] = new int [args.length];
        int b;
        for(int i=0;i<args.length;i++){
            a[i] = Integer.parseInt(args[i]);
        }
        b = fact(a[0],a[1]);
        if(b==0){
            System.out.println("error!");
        }
        else{
            System.out.println(b);
        }
    }
    public static int fact(int y,int x) {
        if(x==0 || y==0 || y==x) {
            return 1;
        }
        else if(x==1){
            return y;
        }
        else if(x>y){
            return 0;
        }
        else {
            return fact(y-1,x-1)+fact(y-1,x);
        }
    }
}

